It would take Clinton being replaced as the nominee*, because she is the only reason that Trump has as much of a prayer as he does. Trump vs anyone else would have been case closed by this point.
* assuming she isn't replaced with Bernie Sanders
Actually, this is, probably, almost the best imaginable way of improving Trump's chances. Replacing Clinton, assuming this is not done because she is asssassinated or something similar, means that the Democratic party is going to be severely divided for an extended period of time. Furthermore, Clinton political campaign is well-oiled and ready to go: anybody else would be struggling to improvise. Finally, whom would she be replaced with? The only realistic possibilities this late in the game are Biden or Kaine. Biden is only everybody's favorite grandpa because he is not running. He is, actually, gaffe-prone, and would not be hard to demonize. Kaine, being Clinton's pick, would have to answer for whatever sins that get Clinton replaced.
She would likely be replaced by Tim Kaine, who for no reason would have to answer for emailgate, the Clinton Foundation, or Bill's being a sexual predator. Though very bland, Kaine would also be an infinity% over Hillary in natural political skill department—all in all, I don't think Trump could beat someone with minimum baggage.
